Individual differences in phase shifts of the human circadian system and performance deficit.
At periodic intervals throughout the biological day, assessments were made for a week prior to intercontinental flight yielding a reference of biological time set to the environment of origin, for two weeks at the temporally displaced environment of destination, and for a week following return to the environment of origin. Assessments made included rectal temperature, heart rate, respiratory rate, palmar evaporative water loss, urinalysis, reaction time, decision time, critical flicker fusion, subjective fatigue and well-being, and intellectual facility. The intercontinental flights consisted of east-west and west-east flights which permitted a comparative analysis of bidirectional effects of time zone displacements and a north-south flight which provided an appraisal of effects solely attributable to prolonged flight. Shifts of the phase of circadian periodicity manifested by the physiological functions were effected by the east-west and west-east flights but not by the north-south flight. Bidirectional differences in lag time and extent of dissociation were revealed, but these may be due to individual variability. All flights engendered significant increment in subjective fatigue. Only one flight, the east-west, produced significant performance deficit indicating that while rapid translocation through many time zones does effect physiological phase shifts requiring 4-6 days for completion and impairment of well-being, these phenomena are not accompanied by a commensurate change in the efficiency of basic psychological functions.